1. Field of the Invention
This invention relates to horizontal boilers of the type, hereinafter referred to as the type described, comprising a furnace box from which products of combustion pass, generally horizontally, to a smoke box and in which there may be provided one or more passes of horizontal smoke tubes extending between the smoke box and a further smoke box or boxes. Such a boiler may be of the shell type comprising an outer shell containing the water/steam space of the boiler as well as the furnace box and the or each pass of smoke tubes, if provided.
2. Summary of the Invention
An object of the invention is to provide a boiler of the type described with a fluidised bed in the furnace box to fire the boiler. It has been found that problems arise when a fluidised bed is provided in the furnace box due to the limited height available above the bed which leads to a relatively great carry-over of fluidised material from the furnace to the rear of the boiler into the first smoke box.
Another disadvantage is that the fluidised material tends to migrate along the furnace box in the direction of discharge of products of combustion from the furnace. This is undesirable since the front of the fluidised bed could be left uncovered with subsequent short circuiting of air through the front of the bed to the detriment of the material at the back of the bed.
The present invention provides a solution to these problems.
According to the invention we provide a boiler of the type described wherein the furnace box comprises a lower compartment, means to establish a fluidised bed in the lower compartment and an upper compartment which is connected to an exit for products of combustion from the furnace box, the lower compartment being isolated from said exit except for communication through the upper compartment through a foraminous element.
The provision of the foraminous element causes the products of combustion arising from the bed to be spread across the whole of the bed area and in consequence the upward movement of the fluidised material and products of combustion is substantially reduced in velocity compared to the velocities that would be experienced if the gases left the furnace directly. Furthermore, as a result of providing the foraminous element, all the products of combustion and fluidised material rise vertically; as a consequence migration of the bed towards the discharge end of the furnace box is avoided.
The foraminous element may comprise an apertured plate, which may be made of stainless steel, located adjacent to, and spaced inwardly of, the wall of an upper part of the furnace box and extending over the majority of the length of the furnace box from a forward end thereof towards a rear end thereof and there being a duct extending downwardly from the space between the upper wall part of the furnace box and the foraminous element to a generally horizontally extending discharge duct communicating with the smoke box.
The downwardly extending duct may be bounded at one side by a plate depending downwardly from the foraminous element and having its lower end embedded in a bed of particulate material to provide a seal to said discharge duct.
The furnace box may be generally cylindrical with the Longitudinal axis of the cylinder extending horizontally and the foraminous element may be generally part circular in cross-section, the lower ends of the element being connected to the wall of the box.
The part circular cross-sectional shape may have a centre of curvature coincident with the axis of the furnace box and the lower ends of the element being connected to the wall of the furnace box by horizontally extending elements.
The foraminous element may be perforated with a greater number of holes per unit area at the front of the furnace box compared with the rear of the furnace box so as to minimise the possibility of short circuiting of gas by reducing the pressure drop across the element at the front because of the increased flow permitted.
The foraminous element may be made up of a plurality of plates overlapped longitudinally and transversely to accommodate thermal expansion and the element may be supported from the wall of the furnace box by appropriate struts and thus the element is water cooled as a result of conduction of heat through the struts to the water cooled wall of the furnace box.